Members of transforming growth factor-beta (TFG-beta) superfamily are multifunctional growth factors that control a broad range of cellular responses in metazoan organism. TGF- betas inhibit the growth of most epithelial and hematopoietic cells and regulates the production of extracellular matrix by mesenchymal cells. Loss of the antiproliferative responsiveness to TGF-beta is often considered as a major step in tumor progression. The long-term objective of this application is to understand the molecular basis of how alterations in TGF-beta antiproliferative signaling pathways lead to deregulation of growth control in human diseases. The general strategy of this application is to focus on the mechanism of ubiquitin/proteasome- dependent degradation of tumor suppressor Smad2. Smad2 is an important TGF-beta signal transducer that inhibits cell proliferation though transcription-dependent mechanisms. However, the precise regulation of Smad2 remains enigmatic. The unifying hypothesis of this proposal is that ubiquitin/proteasome pathway targets Smad2 for degradation through a specific ubiquitin E3 ligase. To test this hypothesis, we will study the mechanism of Smad2 degradation by a newly identified ubiquitin E3 ligase called Smurf2 and investigate the physiological consequences of deregulation of Smurf2 activity in Smad2 degradation. Three Specific Aims are proposed: 1. Define the molecular mechanism of Smurf2-mediated Smad2 degradation. The physical interaction between Smurf2and Smad2, the structural domains required for the interaction, and the molecular basis for substrate selectivity will be determined. 2. Investigate the physiological impact of Smurf2-mediated Smad2 degradation in TGF- beta signaling. It will be tested how Smurf2-mediated Smad2 degradation affects Smad2 signaling during TGF-beta induced cell cycle arrest and Xenopus early embryonic development. 3. Examine the regulation of Smurf2-Smad2 degradation in epithelial cells. Molecular and biochemical approaches will be used to define the cellular mechanism of Smurf2-mediated Smad2 degradation and to analyze differential Smad2 degradation in normal and cancer cells. The proposed studies should help to establish a working theory for ubiquitination-mediated Smad2 turnover during TGF- beta-induced growth inhibition and to understand the mechanisms of Smad actions in malignant transformation and progression of human cancers. Finally, the result may provide a foundation of the rational design of novel therapeutic approaches for cancer prevention and treatment.